We need to add support for licensing content uploaded to GMG
instances. For basic support we need to implement the following:


-  attach the license (identified by a URI) to Media
-  allow users to select a license for media (probably start with
   fixed set)
-  display license when users view the media page

I think there should be some documentation for this feature (e.g.
below entry fields) to explain its functionality.

The string at mediagoblin/auth/forms.py:48 ("Your email will never
be published.") may not apply to every instance of MediaGoblin:
someone might want to install MediaGoblin and not promise to keep
the e-mail addresses private.

The string needs to be changed/removed, or made admin-editable.